Method for the precision production of infiltrated articles



' July 15,1958 J. ELLIS ET'AL 2,843,501

. CISION METHOD FOR THE PRE PRODUCTION OF INFILTRATED ARTICLES Filed Aug. 1, 1956 2 Sheets-Sheet 1 FIG. 4.

FIG. '2.

FIG.

JOH/VZ. 4 4/5 Jay 0. KIVO X Ja /A 5 444/15 METHOD FOR THE PRECISION PRODUCTION OF INFILTRATED ARTICLES John L. Ellis, White Plains, and John D.'Knox, Hastingson-Hudson, N. Y., John B. Adarnec, Closter, N. 1., and Claus G. Goetzel, Hastings-on-Hudson, N. Y., assignors to Sintercast Corporation of America, Yonkers, N. Y., a corporation of New York Application August 1, 1956, Serial No. 601,526

9 Claims. (Cl. 117-50) The present invention relates to a method of produc- 7 ing by infiltration articles of closely controlled dimensions in the production of tools, dies, bearings, seals and related wear resistant products and for use in fields involving high temperature systems such as prevail in heat engine power plants (e. g. jet engines, rocket motors, etc.) and other types of high temperature environments involving corrosion, erosion, etc. Recent advances in the design of heat engine power plants have necessitated the. development of heat resistant hard and strong metals capable of withstanding high operating temperatures of up to about 950 C. and higher. However, the production of such materials regardless whether they are used at ordinary or elevated temperatures has always presented difiiculties in view of the relatively high melting points of these refractory compounds which could not be melted and therefore could only be utilized in combination with matrix-forming bonding metals by means of liquid phase sintering.

One method which is employed in producing articles based on refractory compounds, such as titanium carbide, comprises commingling grains of the carbide with a matrix-forming metal or alloy powder followed by pressing intothe desired shape and then sintering the shape at an elevated temperature above the melting point of the matrix metal but below that of the refractory compound (i. e. liquid phase sintering) to effect coalescence through. shrinkage. Another method which is employed comprises producing a porous and coherent skeleton of a high melting substance (e. g. a metal, alloy or compound based on one of the so-called refractory metals tungsten, molybdenum, tantalum, columbium, titanium, zirconium, etc., and characterized by a melting point above 1535 C.) into which the matrix-forming, metal is infiltrated or interstitially cast until substantially all the pores are filled. In either case a composite structure is formed comprising said high melting substance dispersed through the matrix metal. 1

The heating temperatures employed in producing the articles are high and may range up to about 250 C. above the melting point of the matrix metal. Such liquid phase heating or sintering temperatures may range from about 1100 C. to 1700 C. Because of the high melting temperatures and the nature of materials employed, the liquid and solid phase prevailing during the sintering are generally chemically active. and tend to react with the United States atent g environment in which the liquid phase sintering is conducted, with the result that the product is in many instances adversely aflected. This hold true for the prevailing atmospheric conditions, as well as the supports or containers contacting thearticlej Thus, it is important that the environment be maintained as substantially inert or protective as possible. In the case of the atmosphere, inert or protective conditions may be achieved by utilizing inert gases such as-argon, helium, etc., or

by utilizing a reducing gas such as carbon monoxide or hydrogen which are considered protective for the purpose. Substantially inert or protective conditions mayalso be effected by employing such gases at sub-atmospheric pressure ranging down to high vacuum. Generally speaking, the most troublesome variable is the support, whose behavior at elevated'temperatures cannot be predicted with any certainty, especially in metalliferous systems involving a reactive molten metal phase and where the heating takes place under subatmospheric pressure. This problem is particularly acute in the production of heat resistant articles by infiltration involving a mass movement of infiltrant metal through and/ or around a porous skeleton, e. g. titaniumcarbide, while in contact with the support. 7 i

Considerable development work has been initiated with the aim of developing adequate supporting materials to meet particular needs in the aforementioned type of heating. For example, it has been found that substan has been found to have a broader range of utility over the two foregoing ceramic oxide substances. Zirconia has the advantage of insuring more consistently the production of infiltrated articles of good surface quality.

Recent work has indicated similar advantages for thoria of high purity; V t

While it has been found that the foregoing ceramics when used under specified conditions, have resulted in products of improved metallurgical quality, for example improved internal and external quality, practical considerations have necessitated that these products 'be producedas close as possible toblue print sizes inorder to minimize finishing operations, in particular grinding. Articles made of refractory metal substances, such as titanium carbide, are very hard and require the use of special and expensive grinding procedures (e. g. diamond wheel grinding etc.) to produce a finished product. Thus, it is important that size deviations in the product be avoided. as much as possible. The problem is particularly acute in the production of complex shapes, for example a turbine bucket of tapered cross section having a slight twist in its foil section and having at one end a heavy root and possibly at the other end a heavy shroud section, or a nozzle vane that has a heavy cross section near the leadingv edge, a thin cross section at the trailing edge, and possibly a hollow running parallel with the longitudinal axisof the product.

Heretofore, in producing t-ration, the ceramic employed as the support for the skeleton was usually used in bulk as a powder pack.

This was done by partially fillinga cylindrical flask of refractory metal such as tungsten with loose ceramic powder, e. g. zirconia powder, followed by inserting into it gently in an upright position the skeleton (e. g. a

P atented July 15, 1958 such complex shapes 'by infil- 3 turbine blade skeleton with its root section pointing upward) and the entire assembly vibrated on a jolting table to settle and pack the ceramic powder firmly about the skeleton, meanwhile adding more ceramic to it until its surface level is just below or even with the top of the root section, thetop of the root section being left exposed to receive infiltrant metal. During infiltration, the infiltrant metal flows in the skeleton body by gravity under substantially protective conditions.

As has been stated herein, while infiltrated products of improved metallurgical quality have been produced by this method, it was not always possible to produce con: sistently articles close to specification sizes. The use of ceramic powder in bulk surrounding the skeleton was not conducive to good temperature control (by bulk is meant thickness of ceramic of the order of about three quarters of an inch and higher, for example one and a half inches). The heat transfer in such molds was poor and, thus, it was difficult to obtain good uniform heating throughout the mold. The bulk ceramic would shrink non-uniformly whereby cracks would occur in it causing infiltrant metal to leak out and form undesirable fins and crusts on the infiltrated article. Furthermore, the nonuniform shrinkage of the powder pack would cause the skeleton to move and distort within the mold and be subjected to bending stresses. Even when the ceramic did not crack and a product of good surface appearance and properties was obtained, the product was often warped sufiiciently as to make it practically useless for grinding to size.

It has now been discovered that the foregoing difficulties can be overcome by using a special mold structure comprising a rigid mold support in combination with a thin yieldable interface of an inert ceramic substance.

An important object of the present invention is to provide a novel method of liquid phase sintering and in particular infiltration, whereby infiltrated refractory metal, refractory metal alloy or refractory metal compound articles can be produced having high dimensional fidelity in conjunction with improved internal and external metallurgical quality.

Other objects will more clearly appear from the following description when taken in conjunction with the drawing wherein:

Fig. 1 is illustrative of an embodiment of a mold assembly which may be employed in carrying out the method of the invention showing in cross section the arrangement of a titanium carbide turbine bucket skeleton with respect to a ceramic interface and a mold support of graphite;

Fig. 2 is a cross section of a similar mold assembly in combination with a hollow blade skeleton of titanium carbide adapted to carry out the novel method, said section taken through line 22 of Fig. 3;

Fig. 3 is a cross section of the assembly of Fig. 2 taken through line 3-3;

Fig. 4 shows a cross section of Fig. 5 taken through line 4-4;

Fig. 5 depicts a vertical cross section of another mold assembly combination utilizing the novel method in the production of a turbine blade from a titanium carbide skeleton having a thick root portion and a thick shroud portion;

Fig. 6 illustrates a compression block produced in accordance with the invention;

Fig. 7 depicts in cross section a mold assembly employing the novel method for producing the compressed block of Fig. 6, the compression block being shown in the mold as taken along line 7-7 of said figure;

Figs. 8 and 9 illustrate a mold arrangement for producing a hollow mold lining by the novel method.

The results of the invention are achieved by controlling the thickness of a ceramic oxide powder between the skeleton and the mold so that in effect it is employed as an interface, the thickness of the interface being determined by the prevailing infiltration conditions and prefof the mold assembly erably by the configuration of the porous skeleton. Because of its ease of machinability and its relatively high heat of conductivity, graphite is selected as the preferred mold support. The graphite may be either dense or of porous structure and of commercial or high purity grade. Other heat and fusion resistant materials comparable to graphite in heat conductivity and formability may be employed as the mold support, for example, materials having heat conductivities of at least 0.01 cal/sq. cm./ cm./ C./sec., preferably at least 0.05, such as molybdenum, titanium carbide, etc. By utilizing a relatively thin ceramic interface backed up by a rigid unyielding mold wall of requisite heat conductivity, it is possible to heat the skeleton body to the desired infiltration temperature quickly and uniformly throughout before infiltration begins, thereafter enabling the infiltration process to be carried out smoothly and efiiciently to produce products of maximum density. Broadly, therefore, the invention comprises supporting the exposed surface of a porous skeleton against a thin, yieldable interface of ceramic powder backed up by a rigid mold wall of mold material having a heat conductivity of at least about 0.01 (in metric units) followed by uniformly heating said supported skeleton to the infiltration temperature before the infiltrant metal is melted so that when the infiltrant metal is subsequently melted, it will flow unhindered through the uniformly heated skeleton.

An important aspect of this invention is that while the thin ceramic interface is yieldable, it has sufiicient shape retentivity to enable the production of coated products in situ. For example, the porous skeleton body while it is similar in its utility to the wax pattern used in precision casting, differs in that it dos not require the same type of controls. In precision casting, the wax pattern must be produced with an oversize in order to compensate for the shrinkage of the cast metal. In the present invention this is not necessary as the porous skeleton is produced to within specification size of the final product, the dimension being assured by the thin ceramic interface surrounding it. Because of the thinness of the yieldable interface, expansion or contraction is hardly of any consequence on the final dimension of the product. To produce a coated product, the porous skeleton is obtained by sintering at a temperature below that of the subsequent temperature at which it is infiltrated so that when the sintered skeleton is being heated to the infiltration temperature, it will undergo some shrinkage in the temperature range between its previous sintering temperature and the infiltration temperature, thereby causing it to shrink away from the interface leaving a gap which is filled subsequently by infiltrant metal. The coating compensates for the shrinkage of the skeleton and keeps the final product to within the specification size. Conversely, if no coating is desired, the porous skeleton is produced by sintering at a temperature at least that of the subsequent infiltration temperature, whereby when it is heated to the infiltrating temperature, it does not shrink to any great extent. While the skeleton may expand slightly during heating and press against the yieldable interface, tests have shown that after it is infiltrated and cooled, it contracts back to within the specification size. The important thing is that while the ceramic interface is yield-able, it appears to behave like a stiff support by virtue of the backup support of the rigid mold wall.

Depending upon the shape of the article to be produced, the mold assembly may be formed from two plates which are shaped so that when fitted together they define a cavity whose bounding walls conform substantially to the configuration of the skeleton to be infiltrated. The cavity is made slightly larger than the size of the skeleton so that when the skeleton is centered in the mold a space remains between the mold and the skeleton for receiving inert ceramic powder having a thickness which preferably varies in accordance with the configuration a. .5. of the skeleton- Near and at the top of-the mold, where an end portion-of a skeleton to be infiltrated terminates, the, thicknessof the ceramic interface near the region of the end portion, where the liquid infiltrant predominates as areservoir during infiltration, ranges from about' 0.175, to 0.6 inch. This is to. prevent premature melting of the infiltrant while the skeleton is brought up to temperature, the. infiltrant metal being the last to reach the temperatures Adjacent substantially all of the skeleton surface where. the. reservoir isnot present, the cerainic interface thickness ranges from about 0.05 to 0.3 inch, the thickness adjacent the. skeleton surface being faces of the skeletonwill range in thickness from about I 0.05 to 0.15 inch, while at skeleton edges and sharp corners'the space will range in thickness from about 0.125

to 0.25 inch. In practice, it is desired that the thickness of. the ceramic interface near and at the top of the mold shouldrange'from about0.2-to 0.5 inch, while the thick-.

ness adjacent the smooth surfaces of the skeletonshould range from about 0.075 to 0.1 inch and at the skeleton edges and corners from about 0.15 to 0.225 inch.

The filling of the space between the skeleton body and the closely fitting mold with powdered ceramic is achieved by gravimetricv packing, e. g. by subjecting the dry powder during and after the fillingofthe space to vibratory and jolting forces (such as ona conventional foundry compressed air jolting table), whereby the ceramic particles are brought as closely as possible into interlocking relationship with each other, thus minimizing shrinkage of the interface athighfiring temperatures. The density of the gravimetrically packed powder and/consequently its, shrinkage properties may be controlled by controlling the particle size distribution of the powder. This powder should be of asize substantially all less than 30 microns, preferably at; least 90% all finer than 10 microns.

As illustrative 'ofthe infiltration assemblies utilized in carrying out the novel method. provided by the invention, reference is made to Figs. 1 to and 7 to 9.' Fig. l shows a rectangular graphite mold comprising .split sections 1 and 1a held in close-fitting arrangement by graphite clamp 2 which also defines. the bottom of the mold. A titanium carbide turbine bucket skeleton Shaving a root section 4 iscentered in the mold and separated by a space occupied by;a gravimetrically packed powdered ceramic interface 5, e. g. powdered. thoria, which .is thicker at the top of the moldin the region of the root section where the liquid infiltrant metal reservoir predominates than along the sides Supported on graphite ring 6 is another graphite ring 11 internally shelved to support graphite radiation shields 12 a to 14. with a peep hole 15 running through all three.

During infiltration, the infiltr-ant metal is metered bydisc 9 through gate '8 into skeleton 3 .via the root portion 4.

The proportioningof the'refractory, interface thickness along various portions of the skeleton is determined by the. conditions existing fromthe top to thefbottom of the mold.v For example, at the instance-of and during infiltration near the top of the. mold, just above disc 9, a substantially liquid infiltrant reservoir exists which must be protected from the graphite portions of the mold assembly by a relativelythick refractory interface ring 7v and "in the region adjacent the root'section for example about 0.3 inch thick At the smoothsurface of the skeleton in the mold proper-{for example at 16, the interface is maintained at a smaller thicknessgforexample about 0.1 inch thick, as there is-l'ittle build up of liquid phase on the surface of the skeleton. However, at relatively sharp. skeletoncorners where surface tension has a marked affect on causing small pools of infiltrant metal to accumu late, slightly thicker interfaces are necessary, for example of the order of about 0.2 inch. By designing the mold V to meet these conditions, situations which give n'se to shrinkage and warpage of the skeleton are either completely avoided or greatly minimized. Thus, smooth surfaces of the skeleton (e. g.smooth surfaces of an air foil section). are maintainedas free as possible from warping stresses, making it feasibleto produce an infiltrated productclose to specification size; V I

The proportioning of the ceramic thickness accordance with the foregoing concept h-asthe further advantage of enabling the skeleton to be thoroughly and uniformly heated to a temperature at or above the melting point of the infiltrant metal prior to the actual melting of. the metal by the increased insulating effect of the greater thickness of ceramic in the region of the infiltrant metal. V V

The proportioning of the ceramicthickne'ss also has the advantage of enabling the infiltrated product to be controllably cooled in one directiomwhichinthe casev of the mold of Fig. l is from thebottom up, thus insuring a dense structure free from shrinkage tears and. cracks, porosity, pipes and certain gas-produced defects common inprecision castings of similar configurations. By having a plurality of spaced fins 12, 13 and 14 covering the mold, heat loss by radiation. is markedly inhibited so that the top part of the mold is last to cool. 1

, Figs. 2 and 3 illustrate a mold assembly set-up employed in producing a hollow nozzle vane 17" of titanium carbide centered in a one-piece graphite flask 18'having a graphite plug 20 forming'the bottom. -'The centrally located hollow nozzle vane skeleton L7 is surrounded by a substantially inert ceramic 21, e. g. thoria, the thickness being proportioned in accordance with'the inventive concept, for example about 0.1 inch along the smooth surface of the airfoil section, about 0.2 inch at the skeleton leadfiltrant metal box 23 in which is containedbulk infiltrant metal 24 of the same composition.

Figs. 4 and 5 show the type of mold assembly employed in producing a turbine bucketv 25 comprising an airfoil section. having a thin, tapered ftwistedfcross. sec-. tion, the blade having, as shown in Fig.5,- a relatively large rectangular root section 26 at one end anda'rela tively largeshroud ring section 27 at the other end. As in Fig; l, the blade skeleton is centered within a graphite mold comprising sections 28 and 29 internally configu rated to enable the skeleton to be centered therein 'and leave a space of proportioned thickness to receive ce ramic 30 of powdered thoriae The mold rests on graphite bottom 31 and the two sections held together by a graphite clamp not shown or by other suitableholding means. The root section 26fis topped bya porous'titanium carbide gate 32' which in turnsupports. infiltrant metal 33. a Y

.The reservoir for the liquid metal may be designed in a manner that upon freezing it becomes an integral part of the infiltrated body and. can be. incorporated in machinable ductile fastening portion."

The thin refractory powder interface which is contoured to follow the surface configuration of the skeleton is relatively thick in the region of the root section (for example, between 0.25 and 0.3 inch), relatively thinner along the concave and convex surfaces of the foil section (for example, about 0.1 to 0.125 inch), about 0.2 inch at the leading edge and the trailing edge (Fig. 4), and also about 0.2 inch in the region surrounding the shroud section. By previously employed infiltration methods such blade configurations were almost impossible to produce free from tears or cracks at the junctions between the heavy shroud and root sections and the thin foil section.

Fig. 6 is a three-dimensional view of a compression block of a high carbon, titanium alloy tool steel produced in accordance with the mold arrangement illustrated in Fig. 7 which shows a cross section taken along line 7--7 of Fig. 6 of porous titanium carbide skeleton 34 in graphite mold 35, the top of the skeleton block supporting infiltrant metal 36 of carbon steel described in Example IV hereinafter. The thickness of the ceramic interface contoured about the skeleton is substantially uniform (about, for example, one-eighth of an inch) while in the region of the infiltrant metal reservoir the ceramic interface is thicker, for example of the order of about a quarter of an inch.

Figs. 8 and 9 illustrate an advantage of the mold assembly of the invention in the production of a thin walled bushing comprising a hard metal composition of tungsten carbide and cobalt. Details of this mold arrangement are discussed in Example V.

Results of tests conducted with close fitting graphite molds with a proportioned interface of ceramic oxide shows that marked improvements are possible over molds in which ceramic oxide is employed in bulk. In producing a series of infiltrated titanium carbide slabs of approximately one quarter inch thick, 2 inches wide, and 4 inches long, changes across the various dimensions were maintained small and substantially uniform, and at an average of between about 1 /2 to a little over 2 /2 as shown in the following table:

Considering that the skeletons before infiltration are generally about to porous, the foregoing data show the dimensional control which is possible after infiltration when the ceramic oxide interface is proportioned adjacent the walls of the graphite flask in accordance with the inventive concept. When the ceramic oxide is used in bulk as a skeleton support, dimensional deviations in the same product may range from 1% to as high as 10%, even though the product might be metallurgically sound. This non-uniformity is characteristic of bulk molds containing large amounts of powdered ceramic oxide of thicknesses of the order of about three-quarters of an inch and higher.

In order to achieve the results of the invention, it is important that the liquid phase heating be conducted under substantially protective conditions which may be obtained in an inert atmosphere of argon, helium, etc., or of carbon monoxide, hydrogen, or other reducing gases which do not react adversely with the materials 8 employed in the process- It is preferred that the heating be conducted in a technical vacuum of less than 500 microns of mercury column down to about 1 micron or lower for efficient infiltration, the lowest possible attainable vacuum being preferable.

Ceramic oxides which are preferably applicable as inert interfaces in the invention are thoria, beryllia and zirconia, although alumina may also be employed under certain conditions. Certain other types of ceramic-like substances such as boron nitride alone or mixed with the foregoing ceramic oxides have also been found applicable.

The desired qualities of an infacial substance are: (1) it should not react adversely with the skeleton body, the matrix metal, or with the mold material; (2) it should have a melting point higher than the operating temperature employed; (3) it should be chemically and physically stable and have very low vapor pressure at high temperatures; (4) it should be in powder form, should be free flowing and easily packed gravimetrically in the small gap between the skeleton and the mold walls; and (5) it should not sinter strongly together and shrink markedly during the infiltration cycle and afifect adversely the dimensions and shape fidelity of the final product.

While it is known that ceramic oxide interfaces have been employed as a mold wash in the prior art, the interface produced by gravimetric packing employed in this invention is not the same. A ceramic mold wash starts off with a solution binder and once it evaporates, especially during heating, the ceramic layer tends to flake off, chip and abrade away easily thus allowing the liquid metal phase to get underneath the coating and react adversely with the mold. A dried ceramic mold wash cannot take sudden heat shock the way a gravimetrically packed ceramic interface does. Unlike a mold wash layer a dry powder packing is capable of tolerating slight interparticle movement without adverse effects on the cohesiveness of the interface. It would be difficult to produce a mold wash interface of proportioned thickness on two halves of a graphite mold and expect them to match evenly when fitted snugly about a skeleton. Assembling a thus-coated mold about a skeleton is certain to disrupt the mold wash layer. Minor variations in the dimensions of a skeleton are compensated for by the packing technique of the invention, whereas a mold coated with a wash would have to be remachined for each variation. In other words, the interface provided by the invention retains all the advantages of a packed powder and yet acts as an integral part of the mold stuface.

As illustrative of the invention, the following examples are given:

Example I In producing a turbine bucket with a twisted and tapered airfoil section in accordance with the invention, a batch of titanium carbide powder of substantially less than 10 microns in particle size and containing approximately 79.1% titanium, about 19.2% combined carbon and about .14% free carbon (the balance free titanium, iron, oxygen, nitrogen, zirconium, etc.) was blended dry with about 1% by weight of a thermosetting phenolformaldehyde type resin. The mixture was then moistened with acetone, wet mixed thoroughly, and the powder mass finally dried, pulverized and passed through a mesh screen (U. S. standard). Approximately 700 grams of the powder were pressed into a rectangular block (1 /2 inches, by 2 inches, by 5 inches) to a density of about 52% of full density (i. e. 48% porosity by volume). The block was then sintered for one hour at i300 C. in a vacuum ranging from 32 to 25 microns of mercury column. The sintered block with a density of about 60% of full density was cooled under vacuum, removed and then accurately machined to the contours the .cavitybeing slightly.v larger than that of the blade dimensions, the space between the skeleton and the mold being. then packed'with'pulverulent thoria which had been previously treated to drive off any moisture, the powder having the following approximate size distribue10. which had not infiltrated the skeleton body remained above it in the pressed zirconia ring.

The bucket produced in thismanner had an average density of about 6.2 grams per cubic centimeter. The

concave and convex faces of the airfoil section required only polishing before use, whereas the root section re-.

quired machining of serrations-into the excess metal at the root, needed for attaching to the turbine wheel. The

dimensions of the bucket were uniformly controlled Within the tolerance of the design specification.

Example II V In producing a prismatic fluid guide vane with an airfoil shaped hollowrunning parallel to the longitudinal axis in accordance with the invention, a batch of titanium carbide powder of substantially less than 10 microns in particle size and containing approximately 0.14% free U Percent. Less than30microns "About 100 'Less than microns- -About 99 Less' than 15 microns About 98 Less than 10 microns About 95 Less than 2 microns"; About 85 Less than 1.5 microns; About 36 In other words, substantially all of the powder was less than 30, microns. The thickness of the ceramic interface near the infiltrating end or root section was about 0.3

inch, at the smooth surface of. the foilsection about 0.1

inch, and at the edges and corners of the skeleton about 0.2 inch. The entire assembly was vibrated on a conventional factory jolting' table. Additional thoria was 1 added until it completely surrounded the bucket skeleton shape, leaving the endsurface of the root section exposedI 'A porous'tit'anium carbide disc and a titanium.

carbidelgate both of which had been pressed from a minus 325 mesh titanium'carbide powder and sintered in vacuum at 1400 C. 'were'placed on the exposed end surface of the bucket, as shown in Fig. 1, the gate being placed between the disc and the skeleton. A presse'cl'ring of ceramic fitting this disc; was now placed over it and the infiltrant allotment placed into the container. The infiltrant metal in this case consisted of approximately 250 grams of a heat resistant alloy of the type comprising about 14% chromium, about 7% iron, 2.5% titanium,

0.7% aluminum, and the balance substantiallynickel. I The entire assembly was then lowered into a carbon tube induction heated vacuu'm' furnace, the furnace being evacuated very slowly to avoid any. disruption of the thoria interface by the eruption of entrapped air. The skeleton was thoroughly and uniformly heated to a temperature at or above the melting point of the infiltrant ,metal prior to the melting and subsequent infiltration v of'the' molten infiltrant metal into the skeleton. The interstitial penetration or infiltration of the alloy-into the pores of the skeleton was conducted at 1400 C. for one hour at a vacuum of 30 microns of mercury, column at the infiltration temperature. The infiltrant metal penetrates the pores of the skeleton vertically, filling the body completely through its entire length. Directional solidification was: achieved in the infiltrated body by cooling from the-bottom upward, the power input beingpro gressively decreased in the lower portionsv of the furnace while maintaining a high power input at the .upper portions.

, The use of the difierential power input during the cool- Iing caused the infiltrated body to freeze progressively ifrom the extreme end opposite the molten metal reservoir, the conditions being such that the molten metal reservoir was the last portion of the composite body to solidify.

.After cooling in a vacuum to the solidification of the carbon was mixed with approximately 3% paraffin in solution with carbon tetrachloride. powder was passed through a 30 mesh screen. Approximately 300 grams of the powder was loaded into a rectanguiar. rubber bag.% in. by 2% in. by 6 in. in which was positioned a steel core rod of the dimensions and contours of the hollow desired in the nozzle vane. The bag was sealed and thewh'ole assembly subjectedto a hydrostatic pressure of approximately of a ton per square inch. This action compacted the powder to the form, which upon removal from the bag and the withdrawal of the steel core rod, was a reasonablystrong by 2 by 5" briquet with an airfoil shaped hollow 1 wide and a maximum height of 75 running the length of the briquet along thelongitudinal axis. The briquet was sintered at 1350 C. for hour in a vacuum ranging from 10 to microns of mercury column and has the final. density of approximately 58% of full. The

sintered briquet was accurately machined to the required contours of the nozzle. vane, the concave and convex faces being accurately located in relation to the airfoil shaped hollow. The porous hollow titanium carbide v nozzle vane (weighing approximately 100 grams). of

essentially the same dimensions of the required finished product was 2% in. wide, /2 in. highat its maximum cross-sectional height, 5 in. long, andhas' a minimum wall thickness of .l' ini'between-the outside surface and the hollow core at the leading edge and the concave and convex faces.

- The skeleton body was then positioned. into'a' closefitting degassed graphite flask of the type shown in Fig.v 3 having a cavity conforming substantially in shape to the outside contours of the vane, the cavity being slightly larger than that of the vane dimensions. The space between the mold and the skeleton and the hollow of the skeleton was gravimetrically packed with thoria powder,

which had been treated to drive off any moisture,'and

having the same approximate size distribution as given in ExampleI. I

The thickness of the refractory interfaceacross the bot torn edge of the vane was about 0.25 in., at the concave and convex faces about.0.l in., at theleading andtrailing edges of. the skeleton about 0.2. in. (Fig. 3). The entire assembly was vibrated on a conventional factory jolting table." The top end of the skeleton protruded from the thoria approximately' /s in. The infiltrant charge of. approximately. grams consisted of sheet bent to form a rectangular box and slugs fitting inside of a heat resistant .alloy of the type comprising about 14% chromium, about 7% iron, and the balance substantially nickel. The alloy box and slugs were placed overthe exposed surface of the skeleton, and additional thoria packed around the infiltrant. The top of the mold was so shaped that at least 0.2 in. of ceramic was betweenv infiltrant and graphite surface, The entire assemblywas then lowered into a carbon tube induction heated vacuum furnace. The furnace was evacuatedvery slowly'to After drying, the

avoid any disruption of the thoria interface by eruption of entrapped air. The furnace was brought up to the infiltration temperature of 1450 C. at a controlled rate to assure complete and uniform heating of the mold assembly. The infiltration of the alloy into the pores of the skeleton was conducted at 1450" C. for /2 hour at a vacuum of 1 to 10 microns of mercury column. The infiltrated body was cooled from the bottom up by progressively decreasing the power input to the lower portions of the furnace while the temperature was slowly reduced to the solidification temperature of the lowest melting phase. The coo-ling was continued under vacuum until the assembly reached a temperature of 50 to 200 C. The infiltrated vane could be easily removed from the thoria since substantially no reaction occurred between any of the metalliferous phase and the inert support mass, and the thoria had remained in a granulated unsintered state.

The vane produced in this manner has an average density of about 6.4 grams per cubic centimeter and was dimensionally accurate within the required tolerances. The excess metal was removed from the top end by slicing, and the concave and convex surfaces needed only polishing before use.

Example III In producing a turbine bucket with a thin, twisted and tapered airfoil section, with a large rectangular root at one end and a large rectangular shroud ring section at the other, a skeleton of the desired shape and dimensions was produced from a sintered block with the method outlined in Example I.

The machined titanium carbide skeleton weighing 66 grams and having a density of approximately 60% of full was positioned in a two-part, close-fitting degassed graphite flask of the type shown in Fig. 5 having a cavity conforming substantially to the bucket in shape, the cavity being slightly larger than that of the bucket dimensions. The space between the skeleton and mold was then packed by the aid of a jolting table with thoria powder of essentially the same physical characteristics as that used in Example I. The thickness of the ceramic interface at the faces of the root section was approximately 0.25 in., at the faces of the shroud section about 0.2 in., and at the airfoil section about 0.2 in. at the leading and trailing edges (Fig. 4) and about 0.1 in. at the concave and convex faces with the exception that for a length of 0.3 in. along the airfoil, from the transition from airfoil to the rectangular sections the thickness of the interface was .05 in. greater than the distance the rectangular sections projected from the airfoil faces. This configuration of the ceramic interface held the body rigid in the transverse direction at both the airfoil and end sections, but allowed for shrinkage of the body in the longitudinal direction by the cushioning and compressibility efiects of the extra thickness of the thoria powder at the regions of marked changes in cross section.

A porous titanium carbide block or gate of 60% density with the same cross sectional dimensions as the root section of the bucket, and about A; in. high was placed on top of the exposed root surface. A 92 gram slug of a heat resistant metal alloy comprising nickel, about chromium, and about 7.5% tungsten, and about 0.5% carbon, and the balance substantially cobalt was placed on top and thoria was packed around the four sides of the pieces. The thickness of the interface between infiltrant and the graphite flask was about 0.2 in.

The same conditions were employed in the infiltration of the porous bucket skeleton as given in Example I, excepting that the time at infiltration temperature was 20 minutes.

The infiltrated bucket produced in this manner has an average density of about 6.2 grams per cubic centimeter and was dimensionally within the required tolerances. The concave and convex faces of the airfoil section requiredonly polishing before use, whereas the root and shroud sections required the machining of serrations,

In producing a compression block with an I-shaped section atright angles with a rectangular section, approximately 70 grams of minus 325 mesh titanium carbide powder'containing 79.1% titanium, about 19.2% combined carbon and about 2.5% free carbon was compacted in a rectangular steel die. The resulting block was sintered at 1340 C. for one half hour in a vacuum in the range of 10 to 50 microns of mercury column. The sintered block,. 1 in. by 1 in. by 1 /2 in., with a density of about 63% of full density was cooled under vacuum, removed and then accurately machined to the shape of the required compression block (note Fig. l), the I-section being about 1 /2 in long, in. wide at the web, 1 in. wide at the flanges, and /2 in. thick, with a rectangular section /2 in. long by A1 in. thick and 1 in. wide extending at right angles of one of the flanges. The shaped skeleton with I-shaped horizontal and rectangular section vertically downward was positioned into a close fitting graphite mold (Fig. 7) the cavity of which was slightly larger than the dimensions of the block. Thoria powder of essentially the same physical characteristics as that used in Example I was packed in the space between the skeleton and the mold.

Approximately 0.1 in. of ceramic interface separated the skeleton from the mold on faces. The infiltrant metal, in this case a steel with approximately 0.8% carbon, about 0.75% manganese, and the balance substantially iron, in the form of a 50 gram slab was placed on the exposed surface of the skeleton. The thickness of the thoria interface adjacent the infiltrant metal was about /4 inch. The whole assembly was placed in an induction heated vacuum furance. The infiltration of the alloy into the pores of the skeleton was conducted at 1500 C. for /2 hour at a vacuum of 10 to microns of mercury column at the infiltration temperature.

The infiltrated body was cooled directionally towards the infiltrating end under vacuum, stripped from the mold, and polished on all surfaces with the excess metal infiltrant being machined away from one face. The finished block weighed grams and had a density of 6.7 grams per cubic centimeter.

Example V In accordance with this invention, a thin walled bushing for use as a powder metallurgy compacting die liner was produced of tungsten carbide-cobalt hard metal.

Approximately 950 grams of tungsten carbide powder of the size range of 1 to 5 microns was mixed with approximately 2% by weight of parafiin wax in a carbon tetrachloride solution, dried, and passed through a 50 mesh screen, The powder was hydrostatically compacted around a steel core into a hollow cylinder approximately 6 in. high, 2 in. outside diameter, and 1% in. inside diameter. The compact was sintered at 1425" C. for one hour in a vacuum ranging from 10 to 50 microns of mercury column and had the final density of approximately 57% of full.

The sintered carbide cylinder 37 (note Figs. 8 and 9) was located in a degassed graphite cylindrical mold 33 having an inside diameter approximately 0.2 in. greater than the outside diameter of the tungsten carbide skeleton, and a graphite core rod 39 with a diameter approximately 0.2 in. smaller than the inside diameter of the skeleton. The spaces at the bottom and side thus formed were filled with thoria powder 40 of essentially the same physical characteristics as that used in Example I. 'The top surface of the cylinder was exposed. The interfacial thickness of the gravimetrically packed powder along the cylindrical surface of the thin walled skeleton was maintained at above one eighth of an inch, the

inside wall before use.

-WC; 65%" Co) metal in'the form of a cast ring 41' of about 2 /2 in. outside diameter and 1 in. inside diameter :were placed in contact with the exposed surface of the tungsten carbide skeleton.

The entire assembly was lowered into a ca'rbon tube induction heatedvacuum furnace. The infiltration of the alloy. into the pores of the skeleton was conducted at 1480" C; for one-half hour at a vacuum ranging from 10 .to'10'0'microns" at the melting temperature and thereafter directionally cooled.

j The bushing thus produced was sound in all sections,

had an average density of 14.0 gramsper cubic centimeter,

was straight, true, round and concentric, and had a surface which only required a minimum of finishing on the I Example VI In accordance with the invention, a rectangular. test bar was produced of nickel alloy infiltrated molybdenum skeleton.

Approximately 45 grams of minus 325 mesh molybdenum powder was compacted in a steel die into a porous barapproximately 4 along by /2 in. wide by A in. high. Th'e compact'was sintered at 1500 C. for one half hour 'in 'a vacuum ranging from 10 to 50 microns'o'f mercury column and had a final density of approximately 61% er full- J l The sintered porous bar was located with its long axis vertical, ina close fitting'graphite mold, the cavity of which was a little greater than the dimensions of thebar. Thoriapowder of essentially the same physical characteristics as that used in Example I was packed in the spacebetween the skeleton and the mold, the top of the skeletonbeing left exposed. e I Approximately 0.1 inch of thoria interface separated the skeleton from' the mold on the faces, th'e distance being slightly larger at the corners. The infiltrantmetal,

in 'this case a'nickel alloy containingapproximately 5% aluminum-95% nickel in the form ofa 34 gram slug, was

' placedon the exposed surface of the skeleton. The thickness of'the thoria interface adjacent to the infiltrantmetal was about /2 inch. The whole assembly was placed in anti'nduction heated vacuum furnace. The infiltration of the alloy into the poresof the skeleton was conducted at l500 C. for ten minutes at a vacuum of to 80 microns of mercury column at the infiltration temperthoria pack, and polished on all surfaces with the excess infiltrant metal being machined away frorn' one end. Thefinished bar weighed 66' grams and had a density of 95 grams per cubic centimeter.

' Example VII rectangular test bar was produced in'accordance with the' invention comprising'a nickel alloy infiltrated tungsten chromium a lloy'skeletonn V v v Eighty parts by weight of a minus 325 mesh tungsten powder and twenty parts byw'eight of a minus 325 mesh chromium 'powder'were thoroughly blended and "charged into analumina crucible and hca-tedin a reducingatrnosphere toa temperature of 1700" C. for a period 'ofabout one hour. The resulting sinteredand alloyed cake was crushed, pulverized, and'passed through a 325 mesh sieve. l 1 V 'Approximately' 65: grams of the alloy powder were mixed with approximately 3% paraffin wax in a carbon tetrachloride solution, dried and passed through a 50 mesh screen. The powder was compactedin a steel .die into a porous bar approximately 4 inches long by /z: inch wide by A inch high/ The compact was'sintered at Approximately 0.1 inch of thoria interface separated.

the skeleton from the mold on the faces, the distance being a little greater at the corners. The infiltrant metal, in this'case a nickel alloy containing approximately 16% chromium, 17% molybdenum, 4% tungsten, 5% iron, balance substantially nickel, in the form of a 61"gram slug, was placed on theexposed surface of the skeleton. The thickness of the thoria interface adjacent to the infiltrant metal was about /t'"inch. The whole' ass'embly was placed in an induction heated vacuum furnace. The infiltration of the alloy into the pores of the skeleton was conducted at 1470 C. for five minutes at a vacuum of 10 to microns of mercury column at the infiltration temperature.

The infiltratedb ody was cooled directionally toward the infiltrating end under vacuum, removed from the 'thoria pack, and polished on all surfaces with the excess infiltrant metal being machined away from one end. The finished bar weighed 106' grams and had a density of 11169 grams per cubic centimeter. 1

Example VIII 7 In accordance with the invention, a rectangular test.

bar was produced of cobalt alloy infiltrated molybdenum disilicide skeleton.

About 63.2 parts by weight of a minus 325 mesh molybdenum powder and 36.8 parts'by weight of a minus 325 mesh silicon powder were thoroughly blended. The I mixed powder was compacted into s-lug s and charged into a zirconia crucible and heated in a reducingatmosphere" to a temperature of 1040 C., at which temperature a reactiontook place. The resulting reacted slugs were crushed, pulverized, and passed through a 325 mesh sieve.

.1 Approximately 25 grams of the alloy powder were mixed with approximately 3% paraffin wax in a' car'- bon tetrachloride solution, dried and passed through a 50 mesh screen. The powder was compacted in a steel die into a porous bar approximately 4 inches long by /2 inch wide by A inch high. 'The compact was sin tered at 1300 C. for /2 hour in a vacuum ranging from 10 to 50 microns of mercury column and had a final density 'of approximately 66% of full.

The sintered porous bar was located with its long axis vertical in a close fitting graphite mold, the cavity of which was a little greater than the dimensions of the block. Thori'a powder of essentially the same physical characteristics as that used in Example I was packed in substantially Co, in the form of a 25' gram slug, was' placed on; the exposed surface of the skeleton. The thickness of the thoria interface adjacent to the infiltrant metal was about inch. The whole assembly was placed in an induction heated vacuum'furnace. The infiltration of the alloy into thepores of the skeleton was conducted at 1520? C. for 30 minutes at avacuum of 10 to 80 microns of mercury column at the infiltration temperature.

The infiltratedbody was cooled directionally towards the infiltrating end under vacuum, removed from the thoria pack, and polished on allsurfaces with the excess 1 5 infiltrant metal being machined away from one end. The finished bar weighed 41 grams and had a density of 6.97 grams per cubic centimeter.

Example IX Also in accordance with the invention, a test bar was produced of a nickel-aluminum alloy infiltrated tungsten skeleton.

The procedure of Example VI was used except that minus 325 mesh tungsten powder was used to produce a porous skeleton of approximately 46% of full density, the infiltrant metal alloy having a composition of about 68 parts nickel and 32 parts aluminum. The infiltration was conducted at 1800 C. for 30 minutes. The finished bar had a density of 12.1 grams per cubic centimeter.

Example X A test bar was produced of a chromium-containing steel alloy infiltrated chromium boride skeleton.

The procedure of Example VI was employed except that minus 325 mesh chromium bon'de powder was used to produce a porous skeleton of approximately 58% of full density, and the infiltrant metal alloy had a composition of approximately 18% chromium, 0.12% carbon and balance iron. The infiltration was conducted at 1500 C. for one hour. The finished bar had a density of 6.8 grams per cubic centimeter.

One advantage of the invention is that the use of a graphite mold in conjunction with a relatively thin ceramic interface enables the conditions within the mold to be controlled to a large degree that insures the production of metallurgically sound and dimensionally highly accurate products. Due to the good heat transfer properties of the graphite and the minimum of ithermal lag introduced by a thin ceramic interface, such factors as the uniform heating of the mold and skeleton body and the possibility of directional solidification by controlled cooling of the infiltrated products are readily achieved.

The method is particularly applicable to those systems where extensive solubility between infiltrant and "skeleton or low skeleton density, necessitates the presence of a large amount of liquid phase which tends to slump the body.

In the infiltration of an irregular shape with a recessed portion perpendicular to the longitudinal axis, such as a dumbbell shaped tensile or stress rupture test specimen, or a turbine bucket with both a root and shroud ring section, the invention provides 'a method whereby hot tearing due to stresses imposed by the support upon contraction of such dumbbell shaped products during infiltration is overcome. The use of thoria powder in a fluid-like pulverulent or granulated unsintered state as an interface offers the unique property of allowing the object freedom of movement in the longitudinal axis (provided proper thickness of the interface at the transition between the cross sections is maintained), but movement in the transverse direction is restricted by the bolstering effect of the close proximity of the graphite mold wall.

For consistent results, tests have shown that the particle size distribution of the refractory interface should preferably be controlled over the following range:

Percent Less than 30 microns 100. Less than 20 microns"; About 95 to 100. Less than microns About 93 to 99. Less than 10 microns About 90 to 98. Less than 5 microns About 75 to 95. Less than 1.5 microns About to 50.

While the illustrative examples given hereinbefore are mostly concerned with liquid phase sintering (infiltration) in metalliferous systems comprising titanium carhide and a liquid matrix-forming metal, it will be appreciated that other refractory substances may be employed. Such refractory substances are characterized by melting points above 1535 C. and include such refractory metals as tungsten, molybdenum, columbium, tantalum, titanium, zirconium, and mixtures of at least two of these metals with each other and alloys thereof with chromium and vanadium. The expression refractory metal substances as employed herein is also meant to cover compounds of the foregoing refractory metals, as well as chromium and vanadium, such compounds including the carbides, borides, nitrides, silicides, aluminides, and mixtures thereof.

The invention is particularly applicable to refractory metal carbides, particularly titanium carbide or a carbide based on titanium. Thus, titanium base carbide may comprise up to about 5% by volume of each of such metal carbides as silicon carbide, boron carbide, and up to about 10% by volume each of chromium carbide, columbium carbide, tantalum carbide, vanadium carbide, molybdenum carbide, tungsten carbide, zirconium carbide or hafnium carbide, the total amounts of these carbides generally not exceeding 25% by volume of the titanium-base carbide. By titanium-base carbide is meant a carbide comprising substantially titanium.

The matrix-forming metals which may be employed in the metalliferous systems referred to herein include the iron group metals iron, nickel and cobalt, mixtures thereof, and alloys based on these metals, for example, heat-resistant nickel-base, cobalt-base and iron-base alloys as well as a wide range of steels including alloy steels and tool steels. All such matrix-forming metals have melting points above 1100 C.

Examples of nickel-base matrix-forming alloys include: nickel and 20% chromium; 80% nickel, 14% chromium and 6% iron; 15% chromium, 7% iron, 1% columbium, 2.5% titanium, 0.7% aluminum, and the balance nickel; 28% cobalt, 15% chromium, 3% molybdenum, 3% aluminum, 2% titanium, and the balance substantially nickel; 13.5% cobalt, 20% chromium, 4% molybdenum, 3% aluminum, 3% titanium, and the balance substantially nickel; 58% nickel, 15% chromium, 17% molybdenum, 5% tungsten and 5% iron; nickel, 4.5% aluminum, and 0.5% manganese, etc.

Examples of cobalt-base alloys which may be employed as matrix-forming metals include: 69% cobalt, 25% chromium and 6% molybdenum; 65% cobalt, 25% chromium, 6% tungsten, 2% nickel, 1% iron and other elements making up the balance of 1% 56% cobalt, 10% nickel, 26% chromium and 7.5% tungsten; and 51.5% cobalt, 10% nickel, 20% chromium, 15% tungsten, 2% iron, and 1.5% manganese; 44% cobalt, 17% tungsten, 33% chromium, 2.25% carbon, and the balance other metals such as iron, manganese, etc.

Some of the iron-base matrix-forming alloys include: 53% iron, 25% nickel, 16% chromium, and 6% molyb denum; 74% iron, 18% chromium and 8% nickel; 86% iron and 14% chromium; 82% iron and 18% chromium; 73% iron and 27% chromium, etc. Examples of steels which may be employed as matrix-forming metals include: SAE 1010 steel, SAE 1020 steel, SAE 1030 steel, SAE 1040 steel, SAE 1080 steel, etc. Low, medium and high alloy steels may also be employed, including the following: about 0.8% chromium, 0.2% molybdenum, about 0.30% carbon, and iron substantially the balance; about 5% chromium, 1.4% molybdenum, 1.4% tungsten, 0.45% vanadium, 0.35% carbon, and iron substantially the balance; about 8% molybdenum, 4% chromium, 2% vanadium, 0.85% carbon, and iron substantially the balance; about 18% tungsten, 4% chromium, 1% vanadium, 0.75% carbon, and iron substantially the balance; about 20% tungsten, 12% cobalt, 4% chromium, 2% vanadium, 0.80% carbon, and iron substantially the balance.

The matrix-forming metals or alloys broadly suitable in forming heat-resistant articles may contain up to about 30% by weight of a metal selected from the group consisting of chromium, molybdenum and tungsten, the sum of the metals of said group preferably not exceeding 40%, substantially the balance. being at' least one iron group metal selected from the group'consisting of iron,,cobalt and nickel, the sum of. the iron group metals being preferably at least about 40% by weight of the matrix-forming alloy. If desired, the matrix-forming pounds (e. g. titanium-base carbide) and matrix-form ing metals may be produced over a wide range of compositions. In producing bodies by liquid phase sintering orby infiltration, the refractory metal compound may range from about 40% to 80% by volume (preferably about 45% to 75%) and the matrix-forming metal range fromabout 60% to 20% by volume (preferably about 55% to r Although the present invention has been described conjunction with preferred embodiments, it is to be understood that modifications and variations may be resorted to withoutdepartingfrom .the'spirit and scope of the invention as those skilled in the art will readily understand. Such modifications and variations are considered to be within the purview and scope of the invention and appended claims.

We claim: 1. In a method for producing a precision infiltrated body which comprises producing a sintered porous skeleton body of a high melting point refractory material, supporting the surface of said porous body against a thin yieldable interface of a gravimetrically packed substantially inert ceramic powder of high melting point backed up by a rigid mold surface of heat conductivity of at least about 0.01 cal./cm. /cm./ C./sec., the thickness of the -with a molten matrix-forming metal until the skeleton is completely infiltrated and then cooling said infiltrated body from the end opposite the infiltrating end, the -in' filtrating end portion being the last to cool and solidify.

2. In a method for producing a coated infiltrated body which comprises producing a sintered porous skeleton body of a high melting point refractory material at a sintering temperature below the subsequent infiltration temperature, supporting the surface of the porous skeleton against a thin yieldable interface of a gravimetrically packed substantially inert ceramic powder of high melting point backed up by a rigid mold surface of heat conductivity. of at least about 0.01 cal./cm. /cm./ C sec., the thickness of the thin yieldable gravimetrically packed interface beingso controlled that in the region about an infiltrating end portion of the skeleton where the infiltrant metal predominates the thickness ranges from about 0.175

I to 0.6 inch and adjacent the skeleton surface away from said infiltrating end the thickness ranges from about 0.05

- to 0.3 inch, said latter thickness being less than that where thin yieldable gravimetrically packed interface being so thereby to sinter slighfly the ceramic interface lto shrink the skeleton slightly from .the thinf'ceramic interface to leave a space of predetermined thickness vthe 'ebetvveen,v

infiltrating .said uniformly heated skeleton one end, with a molten matrix formingmetaluntil the skeletonis completely infiltrated and surrounded by. an infiltrant layer, and then cooling said infiltrated body from the .end opposite the infiltrating end, the infiltrating end portion a being the last to cool and solidifywhereby acoate mfiltrated body is obtained. r

3. A method for producing aninfiltrated articleio proved metallurgical quality and dimensional fidelity the infiltration of a porous skeleton at elevated temper-a1 'tures'which comprises providing a molddefining surface,

of a fusion resistant material having a heat conductivity of at least about 0.01 'caL /sq. -cm. /cm. C./sec. whose bounding walls definefa cavity which conforms. ,substamf tially'to the external configuration of said skel e ton; said. cavity being larger than said skeleton whereby thethick 0.6 inch and adjacent the skeleton surface away from said infiltrating end the thickness ranges from about 0.05 to 0.3-inch said thickness being less than the thickness where the infiltrant metalpredominates, centeringpthej skeleton in said mold; filling ;the space of gontrplled' thickness with a substantially inert ceramic powder of high, melting point and firmly packing said powderaby grayi metric settling with an end portion of the skeleton left exposed, applying an infiltrant metal to the top portion of said skeleton and heating said skeleton to the infiltration temperature before the infiltrant metal is melted, continuing the heating until the infiltrant metal melts and penetrates the already heated skeleton, and cooling said infiltrated body from the end opposite the infiltrating end, the infiltrating end portion being the last to cool and solidify. V V

4. A method for producing an infiltrated articleof improved metallurgical quality and dimensional fidelity by the infiltration of a porous skeleton at elevated temperatures which comprises providing a mold defining surface of graphite whose bounding walls define a cavity which conforms substantially to the external configuration of said skeleton; said cavity being slightly larger than said skeleton whereby the thickness of the space between the skeleton and the mold walls near the top of the skeleton where the infiltrant metal predominates at the instant of infiltration ranges from about 0.175 to 0.6 inch, at the smooth surface of said skeleton from about 0.05 to 0.15 inch, and at the skeleton edges and cornersfrom about 0.125 to 0.25 inch, centering the skeleton in saidmold,

' filling the space remaining of controlled thickness with a substantially inera ceramic powder of high melting point and firmly packing said powder by gravimetric settling with the top of the skeleton left exposed; applying an.

of graphite whose bounding walls conform substantially to' the external configuration of said skeleton, said cavity being slightly larger than said skeleton whereby the thickness of the space between said skeleton and the mold walls near the top of the skeleton where the infiltrant metal predominates at the instant of infiltration ranges from about 0.25 to 0.5 inch, atthe smooth surface of said skeleton from about 0.075 to 0.1 inch, and at the skeleton ,ed-ges'and corners from about 0.15 to 0.225 inch, centering the skeleton in said mold, filling the space remaining of controlled thickness with a substantially inert ceramic ,powder of high melting point and firmly packing said powder by gravimetric settling with the top of the skeleton left exposed, applying an infiltrant metal to the top of said skeleton and heating said skeleton uniformly to the infiltration temperature before said infiltrant metal is melted, continuing the heating until the infiltrant metal melts and penetrates the already uniformly heated skeleton and cooling said infiltrated body from the end opposite the infiltrating end, the infiltrating end being the last to cool and solidify.

6.,A method for producing an infiltrated article of improved metallurgical quality and dimensional fidelity by the infiltration of a porous refractory compound skeleton at elevated temperatures which comprises providing a mold-definingr surface of graphite whose bounding walls conform substantially to the external configuration of said skeleton, said cavity being slightly larger than said skeleton whereby the thickness of the space between the skeleton and the mold walls near the top of the skeleton where the infiltrant metal predominates at the instant of infiltration ranges from about 0.175 to 0.6 inch, at the smooth surface of said skeleton from about 0.05 to 0.150 inch, and at the skeleton edges and corners from about 0.125 to 0.25 inch; centering the skeleton in said mold; filling the space remaining of controlled thickness with a substantially inert ceramic oxide powder of high melting pointand firmly packing said powder by gravimetric settling with the top of the skeleton left exposed; placing on top of said skeleton a porous disc of the same material as the skeleton;"applyi'ng a heat resistant infiltrant metal of melting .p'ointabove 1100 C. to the top of said disc and heating said skeleton to an infiltration'temperature ranging from above 1100 C. to 1700 C. under protective conditions before said infiltrant metal is melted, continuing the heating until the infiltrant metal melts and penetrates the already heated skeleton and cooling said infiltrated body from the end opposite the infiltrating end, the infiltrating end being the last to cool and solidify.

7. The method of claim 6 wherein the porous skeleton body comprises a high melting point refractory metal substance selected from the group consisting of tungsten, molybdenum,.columbium, tantalum, titanium, zirconium, mixtures of at least two of these metals with each other, alloys thereof with chromium and vanadium, and their carbides, borides, nitrides, silicides, aluminides and mixtures thereof, and wherein the ceramic oxide forming the interface is selected from the group consisting of thoria, zirconia, beryllia and alumina.

8. The method of claim 7 wherein the refractory substance comprises a refractory carbide.

9. The method of claim 7 wherein the refractory substance comprises tungsten.

References Cited in the file of this patent UNITED STATES PATENTS 2,612,443 Goetzel Sept. 30, 1952 

1. IN A METHOD FOR PRODUCING A PRECISION INFILTRATED BODY WHICH COMPRISES PRODUCING A PRECISION INFILTRATED TON BODY OF A HIGH MELTING POINT REFRACTORY MATERIAL, SUPPORTING THE SURFACE OF SAID POROUS BODY AGAINST A THIN YIELDABLE INTERFACE OF A GRAVIMETRICALLY PACKED SUBSTANTIALLY INERT CERAMIC POWDER OF HIGH MELTING POINT BACKED UP BY A RIGID MOLD SURFACE OF HEAT CONDUCTIVITY OF AT LEAST ABOUT 0.01 CAL./CM.2/CM./*C./SEC., THE THICKNESS OF THE THIN YIELDABLE GRAVIMETRICALLY PACKED INTERFACE BEING SO PROPORTIONED RELATIVE TO THE CONFIGURATION OF THE SKELETON THAT ABOUT AN INFILTRATING END PORTION OF THE SKELETON WHERE THE INFILTRANT METAL PREDOMINATES THE THICKNESS OF THE THIN INTERFACE IS GREATER THAN THE INTERFACIAL THICKNESS ADJACENT THE SKELETON SURFACE AWAY FROM SAID INFILTRATING END PORTION, RAISING THE TEMPERATURE OF THE SKELETON UNIFORMLY TO AT LEAST THE DESIRED INFLTRATION TEMPERATURE, INFILTRATING SAID UNIFORMLY HEATED SKELETON FROM ONE END WITH A MOLTEN MATRIX-FORMING METAL UNTIL THE SKELTON IS COMPELETELY INFILTRATED AND THEN COOLING SAID INFILTRATED BODY FROM THE END OPPOSITE THE INFILTRATING END, THE INFILTRATING END PORTION BEING THE LAST TO COOL AND SOLIDIFY. 